1. Field of the Invention
The present invention relates to a magnetic coupling lens driving actuator.
2. Description of the Related Art
Conventionally, to bring in full play the focusing and zooming functions of an optical system in an endoscope, it was necessary to provide a mechanism to move a lens within the lens tube of the endoscope in the direction of an optic axis. An actuator is generally used to move the lens within the lens tube. The amount of displacement achievable with a typical actuator depends on the length of the actuator. To achieve a longer displacement, a longer actuator is required. However, an actuator that can be installed in a rigid section provided at the end of the endoscope cannot provide the desirable longer displacement. Consequently, the freedom in designing of optical system in an endoscope was restricted.
An example of a structure that can solve the above-mentioned problem, a magnetic coupling actuator has been disclosed in Japanese Patent Application Laid-Open No. 2008-194326.
FIG. 3 is a cross-sectional view of a structure of a conventional magnetic coupling actuator. In the conventional magnetic coupling actuator, a lens holder 903 that holds a lens 902 is made of magnetic material. Moreover, the lens holder 903, along with the lens 902, is encased inside a lens tube 904. The lens holder 903 is movable in the direction of an optic axis 900. Focusing and zooming is achieved by moving the lens 902 in the direction of the optic axis 900.
On an outer surface of the lens tube 904 is provided a permanent magnet 905 so as to oppose the lens holder 903. The lens holder 903 and the permanent magnet 905 are mutually magnetically coupled via the lens tube 904. A shape memory alloy wire 906 is connected to one side of the permanent magnet 905. The permanent magnet 905 moves in a direction that is parallel to the optic axis 900 with the expansion and contraction of the shape memory alloy wire 906.
Following the movement of the permanent magnet 905, the lens holder 903, which is magnetically coupled to the permanent magnet 905, moves in the direction of the optic axis 900. The shape memory alloy wire 906 is encased within a bendable tube 908. Because the tube 908 and the shape memory alloy wire 906 are bendable, they can be encased within a bendable section of the endoscope. Consequently, it is possible to relax the restrictions on the length of the shape memory alloy wire 906. Moreover, it is possible to use the shape memory alloy wire 906 of a length that can produce the amount of displacement that is sufficient for moving the lens 902.
As shown in FIG. 4, in order to move the lens holder 903 by using the permanent magnet 905 (attractive force F (horizontal component Fh and vertical component Fv)), the permanent magnet 905 must be first moved to a position where the horizontal component Fh of the attractive force (adsorption force) F and a static frictional force Fs of the permanent magnet 905 are counter balanced, and then subsequently moved further. Consequently, the shape memory alloy wire 906 must be expanded/contracted more than the desired amount of displacement of the lens 902. FIG. 4 is a cross-sectional view of the conventional magnetic coupling actuator for explaining the working principle of the conventional magnetic coupling actuator.